1. Technical Field of the Disclosure
The present embodiment is related in general to anchor handling winches, and in particular to a hybrid winch with torque impulse generation control for anchor handling in offshore semisubmersible oil rigs.
2. Description of the Related Art
Floating oil rigs are constructed on floating platforms. Some of these platforms are anchored to the sea bed. The floating platforms are tethered to the anchors with cables. One of the most dangerous jobs at sea is anchoring the floating oil rig platforms. These tethering cables are up to three inches in diameter and can be up to three miles long. Anchor handling vessels set the anchors in the sea bed and winch the end of the tethering cable connected to the anchor to create tension on the tethering cable and to set the anchor in the seabed. The anchor handling vessel pulls on the anchor end of the tethering cable to set the anchor.
The anchor handling vessel resists the pull of the tension placed on the anchor cable with the vessel's propulsion system, typically a diesel engine. If the vessel's propulsion system fails, the pull of the anchor cable can pull the vessel underwater and submerge the vessel, risking the lives of the crew aboard the anchor handling vessel. In addition, if the vessel propulsion system fails, tension on the tethering cable can pull the anchor handling vessel backwards without the benefit of steerage, or the benefit of the active resistance of the vessel propulsion system.
Recent advancements in the art disclose a power-assisted winch including a control system for detecting the amount of turning force or torque supplied by a manual input drive to the winch drum, which supplies turning force or torque from a motor to the winch drum and controls the amount of torque supplied by the motor to the winch drum as a function of the amount of turning force or torque supplied by the manual drive. The function may be a fixed predetermined ratio or it may vary depending upon the level of turning force or torque supplied by the manual input drive. A torque sensor may be utilized to detect the level of manual torque, and a control apparatus will adjust the amount of torque supplied by the motor to the winch drum as required. However, the primary operation of the winch is manual, although it is assisted by power. In the case of semisubmersible oil rigs, manual operation is not feasible due to the heavy duty nature of anchor handling operations.
One of the existing systems for monitoring and controlling the length of anchor cable comprises a winch, a sensor assembly associated with the winch and a control circuit connected to the sensor and the winch. The winch is mounted on a vessel and has a rotatable element about which a length of anchor cable is at least partially wrapped. The sensor assembly is mounted adjacent to the rotatable element to generate signals representing the amount and direction of rotation of the element. The winch controls the rotatable element in a first direction, either in a powered fashion or through free fall, to release the anchor cable from the vessel to lower the anchor. The winch can also rotate the element in a second direction to pull the anchor cable into the vessel to raise the anchor. The control circuit is connected to the winch and to the sensor assembly for determining a length and direction of the anchor cable deployed from the winch utilizing the signals from the sensor assembly. However, there is no backup power source provided to operate the winch in the event of a failure, disconnection or other interruption of the main power.
Another existing device provides a wave motion compensator for a marine winch, in which the tension on the winch rope is maintained substantially constant while the load on the rope moves relative to the winch. This constant tension is maintained by controlling the winch drive motor so that the movement of the winch rope matches the movement of the load without substantial lag. The control is provided by a computer which repeatedly monitors the movement of the winch rope and, by comparing this input data and standard sea-state data, predicts the relative movement of the load and the winch in advance of the time the prediction is made. The computer then issues appropriate commands to the winch drive motor controller. In marine applications involving the lifting or lowering of loads, the computer is also used to determine the optimum time for initiating lifting and completing the lowering of the load, and to automatically perform these operations. However, the device is used for lifting a load from a vessel which is in motion relative to the lifting crane and not for anchor handling. Further, there is no provision for a backup power source if the drive motor power fails.
Various other power-driven winches and systems exist that are used in anchor handling vessels offshore. However, none of the anchor handling winches currently used provides a backup power source for controlled release of the anchor cable when the vessel's propulsion system loses power to prevent the winch from pulling the vessel under water. Without that, cutting the cable attached to the anchor is the only remaining method to prevent the vessel from being pulled under water if power is lost.
Floating oil rigs are constructed on floating platforms that are anchored to the sea bed. The floating platforms are tethered to the anchors with cables. One of the most dangerous jobs at sea is anchoring the floating oil rig platforms. These tethering cables are up to 3 inches in diameter and can be up to 3 miles long. Anchoring vessels set the anchors in the sea bed and winch the end of the tethering cable connected to the anchor to create tension on the cable and to set the anchor in the seabed. The anchoring vessel pulls on the anchor end of the tethering cable to set the anchor. The anchoring vessel resists the pull of the tension placed on the anchor cable with the vessel's propulsion system, typically a diesel engine. If the vessel's propulsion system fails, the pull of the anchor cable can submerge the vessel, risking the lives of the crew about the anchoring vessel. In addition, if the vessel propulsion system fails, tension on the tethering cable can pull the anchoring vessel backwards without the benefit steerage in the reverse direction or the benefit of active resistance of the vessel propulsion system.